In the field of semiconductor fabrication, lift-off techniques are commonly utilized to pattern metal lines and other structures on a surface of a substrate (e.g. wafer). While lift-off techniques are typically not utilized in the manufacturing of VLSI circuits, lift-off techniques can be very useful in early stages of product development and prototyping. A lift-off process is essentially an “additive” technique, as opposed to a “subtracting” technique such as etching. For example, in a typical lift-off process, a layer of sacrificial material is coated over a substrate and then covered by a layer of photoresist material. The photoresist layer is lithographically patterned to form a photoresist mask, and the layer of sacrificial material is isotropically etched via a wet etch process, for example, using the photoresist mask as an etch mask. The isotropic wet etch serves to vertically etch the exposed portions of the sacrificial material down to the substrate, as well as laterally etch the layer of sacrificial material to undercut the photoresist mask and form a photoresist overhang structure. A metal is then non-conformally deposited over the substrate, coving the photoresist mask and filling in the etched openings in the layer of sacrificial material. Due to the photoresist overhang, the deposited metal layer is discontinuous at the undercut regions where the sacrificial layer was removed from under the photoresist mask. This discontinuity in the metal layer allows “lift-off” of the overburden metal on the photoresist mask by dissolving the photoresist mask with a solvent. With this process the overburden metal that is disposed on the photoresist mask is removed, while the metal that was deposited over the exposed substrate remains.
With a conventional lift-off method as outlined above, a thickness of the deposited metal film cannot substantially exceed the thickness of the sacrificial layer. If the deposited metal film is too thick, the metal film would be continuous and prevent the photoresist from being dissolved to achieve lift-off of the overburden metallic material. Furthermore, since the sacrificial material is removed using an isotropic etch process, the use of a thick sacrificial material, which is needed to form a thick metal film on the substrate, will require the formation of a large lateral overhang, since the vertical etch rate of the sacrificial material is the same as lateral etch rate of the sacrificial material. This isotropic etch process effectively places a limit on how close two adjacent metal lines can be formed. More specifically, the minimum distance between two metal lines must be at least 2× the length of the photoresist overhang which, due to the undercutting of the sacrificial material by virtue of the isotropic etch process, is 2× the thickness of the sacrificial material, or otherwise the portion of the photoresist material between the two metal lines would collapse.